Known processes for obtaining containers and trays from a thermoplastic resin foamed sheet include a so-called single side vacuum forming technique in which a foamed sheet is heated to cause post-expansion and softening and the foamed sheet is vacuum-formed by the use of a female mold or a male mold to shape it in a desired formed article. However, the foamed sheet decreases with time and also scatters with changes of conditions of preparation of the sheet. Therefore, formings obtained by this process have a scattering thickness, which leads to insufficient strength or poor fitness when piled up, thus resulting in reduced yield of non-defectives.
In order to solve the problems associated with single-side vacuum forming, double-side vacuum forming of thermoplastic resin foamed sheets has been developed as disclosed in JP-B-59-1184 and JP-A-60-192615 (the term "JP-B" as used herein means an "examined published Japanese patent application" and the term "JP-A" means an "unexamined published Japanese patent application"), in which a pair of forming molds with a prescribed gap therebetween and having a clamping frame at the periphery thereof are used, the male mold being set at 50.degree. to 60.degree. C., a heated and plasticized thermoplastic resin foamed sheet is clamped between the male and female molds, the gap is evacuated from the both molds to thereby adibatically expand the sheet to the full of the gap to give a prescribed shape, the formed sheet is cooled, a gas is introduced into the mold cavity to raise the pressure to atmospheric pressure, the molds are opened, and the formed article is removed. The above-cited references describe that the double-side vacuum forming technique produces formings from a foamed sheet of a thermoplastic resin, such as polystyrene and polymers mainly comprising styrene, or a foamed laminate sheet composed of such a foamed sheet and a thermoplastic resin film.
However, the formings obtained by double-side vacuum forming of foamed sheets of polystyrene or thermoplastic resins mainly comprising styrene are inferior in oil resistance, heat resistance, appearance, and hinge effect as compared with those obtained from foamed sheets of polyolefin resins, such as polypropylene and polyethylene.
On the other hand, a foaming compound of a polyolefin resin, particularly polypropylene, dissipates blowing gas from the surface thereof on extrusion foaming and, as a result, a foamed sheet has low post-expandability, thus failing to provide formings having a desired extent of expansion.